The present invention relates to a transmission control device having a main transmission and a sub transmission.
A conventional type of transmission composed of a main transmission and a sub transmission employs a planetary gear in a speed change mechanism for the sub transmission, the input shaft of the sub transmission being coupled to a sun gear of the planetary gear and the output shaft of the sub transmission being coupled to a carrier of the same gear. The conventional transmission has a clutch for bringing the sun gear into and out of engagement with the carrier and a brake for stopping and releasing a ring gear of the planetary gear, whereby a direct coupling state is attained by engaging the clutch and disengaging the brake while a reduced speed state is attained by releasing the clutch and engaging the brake. Switching between the engagement ahd disengagement of the clutch and the brake is conducted by means of a manual shift valve which is manually operated and, additionally, the switching is conducted by distributing working oil to the hydraulic servomotor of each frictional engagement means for the brake and the clutch by utilizing a low-high switching valve which is controlled by a hydraulic pressure (governor pressure) in response to the rotational speed of the output of the main transmission. Shift control of the main transmission which serves as an automatic transmission is controlled in accordance with the throttle opening and the rotational speed of the output shaft of the main transmission.
Referring to one example of the conventional type of transmission described above, a sub transmission has a speed change mechanism in which frictional engagement means is switched between engagement and disengagement so as to set a variable speed. In this conventional type, during the transient state at the time of changing speeds such as the period of time from the disengagement of the clutch to the engagement of the brake or that from the disengagement of the brake to the engagement of the clutch, the input shaft of the sub transmission (=the output shaft of the main transmission) assumes a free state (a state wherein thereis no transfer of load), so that the shaft of the main transmission speed is not subjected to load. In this state, the ouput rotational speed unavoidably reaches a high revolution during the transient speed change state of the sub transmission, so that the shift control of the main transmission is effected in response to the rotational speed output of the main transmission. Therefore, the prior art transmission entails the risk of involving up shift during the transient speed change state of the sub transmission.